1. Field of the Invention
The present invention relates to PWM power regulators and/or DC—DC converters, and more particularly to a PWM switching DC—DC converter methodology for startup into a pre-charged or pre-biased load.
2. Description of the Related Art
The typical startup sequence of a pulse-width modulation (PWM) DC—DC converter is to ramp the non-inverting or positive “+” input of a feedback control error amplifier from zero (0) Volts (V) to a target VREF setpoint voltage level. The error amplifier is part of a feedback control loop which regulates an output voltage VOUT by comparing the reference voltage, VREF, with a feedback signal based on a sensed portion of VOUT. In an exemplary buck-type converter, the PWM control circuitry provides a PWM signal which controls a gate driver, which further controls a switching device pair to convert an input voltage to the regulated output voltage. The switching device pair includes an upper switching device and a lower switching device, which are typically implemented as a MOSFET pair.
If the converter is started into a pre-baised load, such as when VOUT is already charged, there can be large potentially damaging tranisient currents because VREF is less than the feedback signal. In particular, since the feedback loop attempts to regulate VOUT by comparing the voltage level of the feedback signal based on VOUT with VREF at the input of the error amplifier, where the feedback signal is already high and VREF is ramping up at startup, the control circuit attempts to pull VOUT lower by turning on the “pull down” output switching device (e.g., the lower device of switching device pair). In this situation, the lower device can be turned on long enough to exceed its thermal limit. The surge currents can cause significant stress to on-chip components possibly resulting in catastrophic failure. In addition, these surge currents cause “non-monotinicities” or variations in the output voltage which in some cases can cause downstream integrated circuits (ICs) to latch up and fail.
It is desired to prevent the lower switching device from remaining on too long and to prevent undesirable variations in the output voltage particularly at startup.